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How to Pasteurize Medium and Sterilize
Containers and Tools
Seeds need adequate moisture and optimum
temperatures for germination, but these conditions
also encourage the spread of disease. To increase the
survival rate for seedlings during germination,
containers and tools should be sterilized and
pasteurized or sterile soilless mixes should be used as
growing medium.
Damping-off
One potential problem for gardeners who germinate
seeds at home is damping-off, a fungus disease that
infects seedlings. It is caused by pathogens such as
Pythium, Rhizoctonia and Fusarium and its symptoms
include the browning of stems at the soil line. Stems
then shrivel, plants topple over, and seedlings die.
Infection spreads rapidly, killing most of the seedlings
in a tray or flat. Although it is more of a problem on
over-watered seedlings in poorly drained soils, even
seedlings in medium that drains well can become
infected. Thus, it is important to start clean by
disinfecting all tools and flats, as well as pasteurizing
potting medium or using sterile soilless media to
reduce the possibility of damping-off. For more
information, refer to the fact sheet entitled: Dampingoff of Seeds, Seedlings, and Cuttings
(www.cas.psu.edu/docsCASDEPT/PLANT/ext/
damp_off.html).
Pasteurizing Medium
Potting media mixed by home gardeners should be
pasteurized to kill weed seeds, soil-borne insects, and
pathogens. Pasteurization destroys most organisms
and is done by applying heat until the soil reaches
180oF (82oC) for 30 minutes. When temperatures
are raised above 212oF (100oC), most of the soil
borne organisms are killed and the soil is considered
sterile.
Gardeners can pasteurize homemade potting soils
using a kitchen oven; however, a long-lasting earthy
odor can develop in the oven. To pasteurize soil,
preheat the oven to 180oF. Then fill a pan with about
4 inches of moist soil and cover it with aluminum foil.
Insert a candy or meat thermometer to monitor
temperature and place the pan in the oven. When the
thermometer reads 180oF, leave the pan in the oven
for 30 minutes longer, then remove it and allow the
soil to cool. Seeds can then be sown in the soil, or it
can be used as a transplant medium for seedlings.
Sterilize Containers and Tools
Efforts to pasteurize soil are in vain if contaminated
tools, seed flats, or pots are used. Each should be
disinfected by soaking it for 30 minutes in a 10%
solution of chlorine bleach (one part bleach and nine
parts water). Tools and containers should then be
rinsed and allowed to dry before using them again.
Purchasing Pre-sterilized Potting
Soil or Soilless Mix and Containers
Instead of pasteurizing soil in the kitchen, most
gardeners have chosen to purchase either presterilized, ready-mixed potting soil or sterile soilless
mix. In addition, new plastic or fiber disposable trays
College of Agricultural Sciences • Cooperative Extension
or flats can also be purchased, rather than washing
and sterilizing previously used containers.
Prepared by Phyllis Lamont, consumer horticulture
center library coordinator, Kathleen M. Kelley,
assistant professor of consumer horticulture and
James C. Sellmer, associate professor of ornamental
horticulture
PENN STATE COLLEGE OF AGRICULTURAL SCIENCES
DEPARTMENT OF HORTICULTURE
102 TYSON BLDG.
UNIVERSITY PARK, PA 16802
April, 16, 2003
The Horticulture Fact Sheet series is produced
for home gardeners and professionals by the
Consumer Horticulture Center at Penn State.
The complete series is available on the Web at
www.hortweb.cas.psu.edu.
Visit Penn State’s College of Agricultural Sciences on the Web: www.cas.psu.edu.
Where trade names appear, no discrimination is intended, and no endorsement by
Penn State Cooperative Extension is implied.
Issued in furtherance of Cooperative Extension Work, Acts of Congress May 8 and
June 30, 1914, in cooperation with the U.S. Department of Agriculture and the
Pennsylvania Legislature. T. R. Alter, Director of Cooperative Extension, The
Pennsylvania State University.
This publication is available in alternative media on request.
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© The Pennsylvania State University 2003
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